The invention relates to an operating method for a data bus for several users with flexible timed access. A method of this type is known from International Patent document WO 99/46892A. In the case of such a data bus, a defect may occur in a user, such that the defective user sends data on the bus more frequently than permitted, at unacceptable points in time or continuously and thereby blocks the bus completely and, as a result, may interfere with the data traffic or completely stop the data traffic.
In the case of data buses which operate according to the TDMA (Time Division Multiplex Access) method, as they are known in principle from U.S. Pat. No. 4,866,606 A, so-called bus guardians can be used. These devices have their own clock pulse supply and know the starting and ending points-in-time of the transmission slot of the user to be monitored. As a result, it is possible, by means of the bus guardian, to clear the transmission line of the user only between the starting and ending point in time of the permitted transmission slot and not to allow bus access for the remainder of the time. Thus, in the event of a defect in the user, the interference can only have an effect on the entire bus within the transmission slot of the defective user and not in the remainder of the time (see “Specification of the TTP/C Protocol”, Specification Version 1.0 of Jul. 21, 1999, Document Edition 1.0 of Jul. 21, 1999, Page 113 and on).
In the case of data buses with a flexible, timed access, such an access control cannot be used because of the actual transmission time of the message of the respective user, which is a function of the use of the data bus by the higher-ranking user. This use cannot be predicted from the start, and the actual transmission time therefore varies even from one cycle to the next.
It is an object of the invention to provide an operating method of the initially mentioned type which, also in the case of a data bus for several users with a flexible, timed access, excludes an interference with the data bus traffic caused by a defective user.
This object is achieved by a data bus for several users with a flexible, timed access, characterized in that the users are synchronized, the bus telegrams are emitted in a hierarchical transmission sequence by the users and at least partially only as required and, between the users and the data bus, a switching element is situated which releases the bus access for the respective user only when and as long as the user is permitted to transmit.
The synchronization can take place, for example, by means of a clock pulse generator which emits a synchronization pulse at regular time intervals. In addition or as an alternative, the synchronization can also take place by means of a defined, preferably the highest-priority, communication which is also emitted at regular time intervals.
A special characteristic of the data bus on which the invention is based is the fact that a user will have access to the data bus only when the user actually wants to transmit and is authorized to transmit. If the user does not want to send a message during a cycle, the user, as the transmitter, remains separated from the data bus. A continuously transmitting user, as a rule, has no transmission authorization. The user therefore has no access at all to the data bus. In contrast to the bus guardian in the case of the TDMA bus, the user cannot interfere with the data bus traffic at all.
As known per se, the switching element can have a time control (timing) which is independent of the users. As a result, the point-in-time for the starting and ending of the transmission authorization of the individual user can be independently computed, and an access to the bus at an unacceptable point-in-time, which would be caused by a defect of the clock pulse generator which would have an adverse effect on the timing for the bus operation, can be excluded.
Another advantageous further development of the invention consists of the fact that the switching element has adaptation elements by which the control of the bus access time can be varied beyond the actual transmission time. As a result, for example, construction-specific propagation time delays within the data bus can be taken into account. Details in this respect are contained in German Patent document DE 19810293 A as well as in the following description of an embodiment.
A further development consists of the fact that the variation of the bus access time depends on the function of the user.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.